[llvm] [VPlan] Combine LiveIns fields into MapVector (NFC) (PR #170220)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. As an aside it looks like the only place we need deterministic ordering of the live ins is in `VPlanTransforms::materializeBroadcasts`.

https://github.com/llvm/llvm-project/pull/170220